This invention relates to a molding apparatus of the type having movable mold sections and deals more particularly with a mold and related molding process in which a collapsible core is employed to form a cavity in a molded article wherein interior dimensions of the cavity exceed that of the opening in the article into which the molding core extends.
Containers such as buckets and pails of unitary construction molded from plastic materials are becoming more widespread in use due, in part, to the fact that they may be economically produced in large quantities. Molded plastic containers have replaced many types of previous metal constructions. However, the use of containers for certain products, such as those which are toxic or flammable, require that the container be exceptionally rigid and not subject to rupture or deformation under ordinary use. In connection with this latter requirement, molded plastic containers have not been entirely successful in replacing metal constructions for reasons which will be discussed below. In designing a molded plastic container which is safe for use with paints, thinners and the like, it is absolutely necessary that the lid of the container remain securely fastened to prevent leakage even when the container is roughly handled as during shipping. Although inner locks between the container lid and the rim of the container have been designed to form a liquid-tight seal under ordinary conditions, such seal may be broken and the lid may be inadvertently removed from the container if the rim exhibits flexion.
In order to prevent flexing of the container rim, circumferential reinforcement ribs have been molded integral with the rim in order to increase the hoop strength of the rim. Since it is also a requirement of this type of container that the side walls thereof be free from projections, it is necessary to recess the circumferential reinforcement ribs in the side walls of the container; this in turn requires that the rim be of reduced inside diameter compared to the inside diameter of the side walls of the container.
Because of the reduced rim diameter, it has been necessary in the past to produce the containers using a two-step blow and injection molding process which involves melting a thermoplastic resin; forming a tube from the melted resin using injection molding techniques; and inflating the tube within a blow mold to form the container. This process is undesirable, however, since the tube production steps not only require at least two molds but are also time consuming compared to a one-step process.
Other types of molding processes, such as injection molding, have not been successfully employed in the past to mold containers of the type mentioned because collapsible molding cores of the type necessary for forming the interior side walls of the container have not heretofor been available. The prior art collapsible cores, such as those disclosed in U.S. Pat. Nos. 3,247,548 and 3,660,001, are suitable for forming cavities in relatively shallow articles such as threaded container caps but are not capable of forming deep cavitities such as the interior of a bucket. Collapsible cores of the type disclosed in these patents are formed from a plurality of circularly disposed, resilient molding segments circumferentially surrounding an inner core and rigidly connected to each other at one end thereof. Upon removal of the inner core, the natural resilience of the molding segments causes one end of the segments to spring radially inward, thereby collapsing the outer extremities of the segments to release the molded article. Since only the outer extremities of the molding segments collapse a substantial distance, these collapsible cores may not be practically employed for molding deeper cavitites such as the interior of the container itself.
From the foregoing it is apparent that there is a clear need in the art for a container of the type described above which comprises high density plastic material. It is therefore an important object of the present invention to provide a container of bucket-like configuration having a reinforced rim provided with an interior diameter less in magnitude than the interior diameter of the side walls of the container but yet which is exceptionally rigid and durable.
Another important object of the invention is to provide a one-step process for molding a container of the type mentioned above.
Another object of the invention is to provide die apparatus suitable for injection molding plastic material to form the described container.